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Thermocouple application and principle
Thermocouples are among the most widely used temperature sensing devices in industrial applications. Their advantages include:
1. **High measurement accuracy**: Since thermocouples come into direct contact with the object being measured, they are not affected by any intermediate medium, ensuring accurate readings.
2. **Wide temperature range**: Standard thermocouples can measure temperatures from -50°C to +1600°C. Specialized types, such as gold-iron-nickel-chromium thermocouples, can go as low as -269°C, while tungsten-bismuth thermocouples can measure up to +2800°C.
3. **Simple and convenient design**: Thermocouples are typically made of two different metal wires, which makes them easy to manufacture and install. They are often enclosed in protective sheaths, making them durable and suitable for various environments.
The basic principle behind thermocouple operation involves joining two dissimilar conductors or semiconductors (A and B) to form a closed loop, as illustrated in Figure 2-1-1. When there is a temperature difference between the two junctions, an electromotive force is generated, resulting in a current flow within the loop—this is known as the thermoelectric effect. Thermocouples utilize this phenomenon to measure temperature.
There are two main types of thermocouples: **standard** and **non-standard**. Standard thermocouples follow national and international standards, including the relationship between thermoelectric potential and temperature, allowable error, and standard indexing tables. These are compatible with standard measuring instruments. Non-standard thermocouples, on the other hand, are less precise and lack uniform indexing tables, typically used in specific or specialized applications.
Since January 1, 1988, China has adopted IEC international standards for thermocouples and thermal resistors. Seven standardized types—S, B, E, K, R, J, and T—are now widely used across the country.
The structure of a thermocouple must meet certain requirements to ensure reliable performance:
1. The two hot junctions must be securely welded.
2. The two wires should be well insulated to prevent short circuits.
3. The connection between the compensation wire and the cold junction must be stable and easy to access.
4. The protective sleeve must isolate the hot junction from harmful environmental factors.
To reduce material costs and improve efficiency, especially when using expensive materials like precious metals, **compensation wires** are often used. These extend the cold junction (free end) of the thermocouple to a more stable environment, such as a control room, where the measuring instrument is located. However, it’s important to note that compensation wires only extend the hot junction and do not eliminate the effect of temperature changes at the cold junction. Therefore, additional methods, such as reference junction compensation, are needed to account for variations in the cold junction temperature.
When using compensation wires, it's crucial to match the correct model, ensure proper polarity, and keep the temperature at the connection point below 100°C to maintain accuracy and safety.